The TIROS Operational Vertical Sounder
(TOVS)
equipped aboard NOAA's TIROS series of polar orbiting satellites
consists of three instruments: the High Resolution Infrared
Radiation Sounder (HIRS),
the Microwave Sounding Unit (MSU)
and the Stratospheric Sounding Unit (SSU).
The MSU and SSU have been replaced with improved instruments,
the AMSU-A
and AMSU-B,
on the newer satellites.

One of NOAA's polar orbiting satellites
carrying the TOVS instruments.

Surface radiation received by HIRS channel 9 is
suppressed by 9.7 micron ozone band absorption in the lower
stratosphere such that high lower stratospheric ozone values
directly result in cold HIRS channel 9 radiances. An algorithm
is applied to estimate the 'total' column ozone based upon these
suppressed HIRS 9.7 micron radiances.